Massive MIMO multi-user Beamforming (MM-MUBF) offers the potential to significantly increase the spectral efficiency and throughput by many folds through spatial multiplexing. However, when the number of RF chains and antennas becomes large (It is understood that an antenna is associated with a RF chain, transmit (Tx) or receive (Rx), thus, hereafter when the number of antennas is used, it should be understood to mean the number of antennas and the associated RF chains), there are significant overhead in channel estimation to obtain Channel State Information (CSI). This problem becomes more challenging if the channel coherence time is short, e.g., in the case of a Base Station (BS) with a large number of antennas communicating with a fast moving User Equipment (UE) because a large number of channels need to be estimated frequently, reducing the time left for actual data communication. This problem is further compounded by the number of fast moving UEs. On the other hand, the industry is moving towards Small Cells (SC) and Heterogeneous Network (HetNet) as a way to meet the fast increasing data traffic. Ideally, the placement of SCs should be determined by the data traffic needs, not constrained by the availability of wired connection to the backhaul network. MM-MUBF is a good match for this need as it can provide high capacity wireless backhaul for many SCs so they can be placed at any place there is a power plug. Since the channels between a BS and SCs are very slow varying, the need for frequent estimation of CSI is alleviated.
However, prior art uses out-band wireless backhaul, meaning that different frequency ranges are used for the backhaul between BS and SC and for the communication with UEs. This requires precious frequency resources, which may not be available especially at low frequencies (a few GHz or lower), and even if they are available, they should be used to increase the data throughput with the UEs. In addition, low frequencies wireless backhaul is desired because it does not require line of sight, and offers better penetration than high millimeter wave frequencies. Thus, in-band wireless backhaul, meaning using the same frequencies for the communication with UEs to provide the wireless backhaul between BS and SCs, is desired. This requires a SC to simultaneously transmit and receive in the same frequency channel, referred to as Single Channel Full Duplex (SCFD).
The terms BS and SC will be used to mean either the radio apparatus for transmitting and receiving signals in a cell or the area covered by such radio apparatus, as evident from the context. The term BS is used to mean a cell coverage area much larger than a SC. In the terminology of 4G LTE, both a BS and a SC in this application can be a “small cell”, a SC being a tier below a BS, e.g., a BS below may be a microcell in 4G LTE terminology and SC may be a pico cell in 4G LTE terminology.
This invention presents embodiments that solve the technical challenges discussed above.